Frequency division duplex (FDD) communication systems are communication systems where information is simultaneously sent in both directions, for example from a first node to a second node and from a second node to a first node. To avoid the data transmissions between nodes disturbing or interfering with one another, the two frequency division duplex links work at different frequencies. Examples of systems employing this technique include several 3G cellular mobile telephone systems including UMTS and CDMA 2000.
In such designs the degree of isolation between the transmitter and the receiver is crucial. This is because the receiver has to detect a very weak signal while the transmitter is transmitting a signal that is several orders of magnitude larger in power. This isolation is usually provided by a duplexer possibly working in conjunction with several filters. As the separation in frequency (ΔF) between the transmit frequency FTX and the receive frequency FRX decreases then it becomes increasingly difficult to achieve large isolation and consequently the filter design becomes more complex and requires filters of higher order.
A common approach to reduce the effect of out of band blocking signals (which includes the FDD transmit frequency) whilst minimising the degradation in receiver sensitivity is to use an external filter between a low noise amplifier and a mixer of the receiver. Such an arrangement is schematically illustrated in FIG. 1. The filters can be implemented in different technologies. Some of the most common technologies being surface or bulk acoustic wave filters and dielectric filters. These filters are external devices to the integrated circuit implementing the low noise amplifier and receiver. From the performance point of view this approach is very effective. However this requires the inclusion of pins to connect to this additional component, which in comparison with the integrated circuit is relatively expensive and large.
It is known in super heterodyne receivers to suppress the spurious image responses by use of an on-chip notch filter. The notch filter is often implemented with passive LC networks and is embedded either within the low noise amplifier or in a further transconductance stage of the mixer. It should however be noted that on chip spiral inductors have a relatively low Q factor where in this context Q represents the ratio of the inductive reactance of the inductor to the resistive impedance thereof. As a result, integrated LC notch filters, whilst highly integrated and avoiding the need to provide additional pins to external components, nevertheless have failed to deliver the required performance where the frequency difference between and wanted signal and the blocking or interfering signal is relatively small.